Carrier-signal frequency detector



March 7, 1944. J. J. OKRENT CARRIER-SIGNAL FREQUENCY DETECTOR Filed May6, 1942 .o 52 33 0 o 553%". o -99;

INVENTOR JASPER J. OKRENT BY ATT NEY Patented Mar. 7, 1944 2,848,283CARRIER-SIGNAL FREQUENCY DETECTOR Jasper I. Okrent. Flushing, N. Y.,asslgnor to Hazeltlnc Corporation, a corporation of Dela- ApplicationMay a, 1942, Serial No. 441,895

Claims.

The present invention relates to carrier-signal frequency detectors and,particularly, to frequency detectors which convert frequency variationsof a carrier signal to amplitude variations of the same carrier signalor the signal derived therefrom. While not limited thereto, theinvention is particularly suited for use in a frequencymodulationcarrier-signal receiver to derive the modulation components of areceived carrier signal and will be described in that connection.

In frequency-modulation carrier-signal receivers, it is customary tochange a received frequency-modulated carrier signal at some point inthe receiver to an amplitude-modulated carrier signal to provide a formof carrier signal suitable for detection by a conventionalamplitude-modulation detector to derive the modulation componentsthereof. Such change of form of the carrier signal and the subsequentdetection thereof are efiected in the receiver by a frequency detectorwhich includes a frequency-selective network or frequency discriminatorand means for rectifying the carrier signal of changed form to derivethe modulation components thereof. Sllch frequency detectors in generalare responsive to undesired spurious amplitude variations of thereceived carrier signal, such amplitude variation being due for exampleto atinospheric conditions or electrical disturbances, and, therefore,the frequency detector is sometimes preceded by a limiting system bywhich the undesired amplitude variations of the received carrier signalmay be removed. The use of a separate frequency detector and limitingsystem has numerous disadvantages, for example the increased cost andcomplexity of the receiver, the fact that additional vacuum tubes arerequired with attendant increased maintenance costs, and the increasedpower required to operate the receiver. Both the prior art limitingsystems and frequency detectors have additional limitations individualto each relating primarily to their design and adjustment to effeet theoperation. desired of each.

In order to avoid the disadvantages attendant upon the use of separatelimiting systems and frequency detectors, it has been proposed inaccordance with one prior art arrangement that a frequency detectorhaving somewhat reduced response to undesired amplitude variations of areceived carrier signal be provided by the use of a singlemulti-electrode vacuum tube. In this arrangement. the vacuum tubeincludes two input electrodes and there are derived from the receivedfrequency-modulated carrier signal, and individually applied to thecontrol electrodes, two carrier signals having a relative phase whichvaries with the frequency deviation of the frequency-modulated carrierwave from a predetermined frequency,

bias networks included in the control-electrodecircuits, with the resultthat undesired amplitude variations of brief duration and transientcharacter are not efliciently limited. There is the additionaldisadvantage with this arrangement that the operating characteristic ofthe frequency detector is dependent upon the intensity of the receivedcarrier signal, since it is the intensity which determines'the points onthe operating characteristics of the vacuum tube at which the controlelectrodes are biased. Due to the self-bias feature of this prior artarrangement, the action of the arrangement is so complex that it isdimcult to obtain both linearfrequency detection and effective limiting,especially over a wide range of intensities of the received carriersignal.

It is an object of the present invention, therefore, to provide a newand improved carrier-signal frequency detector which avoids one or moreof the disadvantages and limitations of the prior art devices.

It is an additional object of the invention to provide a carrier-signalfrequency detector which possesses a greatly improved effective limitingcharacteristic and a detector characteristic having a high degree oflinearity.

It is a further object of the invention to provide a carrier-signalfrequency detector of simple and improved circuit arrangement and onehaving an effective limiting characteristic which is fixed by thecircuit parameters and is entirely independent of th intensity of acarrier signal applied to the detector.

In accordance with the invention, a carriersignal frequency detectorcomprises an input circuit adapted to have applied thereto a carriersignal the frequency of which deviates over a predetemiined range offrequency deviation. The detector also includes a vacuum tube includingtwo input electrodes and an anode, and means coupled to the inputcircuit for deriving from the applied carrier signal two carrier signalshaving a relative phase which varies with the frequency deviation of theapplied carrier signal from a predetermined frequency and forindividually applying the derived carrier signals to the inputelectrodes. There is also included in the detector means for effectinganode-current saturation of the tube to render the detectorsubstantially unresponsive to amplitude variat ons of the appliedcarrier signal, and an output circuit coupled to the aforesaid anode forderiving therefrom a signal the amplitude of which varies substantiallyonly with deviations of .the frequency of the applied carrier signalfrom the aforesaid predetermined frequency.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawing, and itsscope will be pointed out in the appended claims.

Referring now to the drawing, Fig. 1 is a circuit diagram, partlyschematic, of a complete frequency-modulation carrier-signal receiverembodying the present invention; Fig. 2 is a graph used as an aid inexplaining the operation of the Fig. 1 arrangement: and Fig. 3 is acircuit diagram of a portion of the carrier-signal receiver of Fig. 1and represents a modified form of the invention.

Referring now more particularly to Fig. there is representedschematically a complete frequency-modulation carrier-signal receiver ofa conventional design embodying the present invention in a preferredform. In general, the

receiver includes a radio-frequency amplifier i having an input circuitconnected to an antenna system Ii, i2 and having an output circuitconnected to an oscillator-modulator 63. Connected in cascade with theoscillator-modulator it, in the order named, are anintermediate-frequency amplifier i l of one or more stages, a frequencydetector i5, more fully described hereinafter, an audio-frequencyamplifier i6 of one or more stages, and a sound reproducer i7.

It will be understood that the various units just described may, withthe exceptionof the frequency detector ill, be of a conventionalconstruction and operation, the details of which are well-known in theart, rendering detailed description thereof unnecessary. Consideringbriefiy the operation of the receiver as a whole. and neglecting for themoment the operation of the frequency detector is presently to bedescribed, a desired frequency-modulated carrier signal is selected andamplified by the radiofrequency amplifier ill, converted to anintermediate-frequency carrier signal by the oscillator-modulator i3,amplified in the intermediatefrequency amplifier i4, and effectivelylimited in amplitude and detected by the frequency detector i5, therebyto derive the audio-frequency modulation. components. Theaudio-frequency components are, in turn, amplified in the audiofrequencyamplifier i6 and are reproduced by the sound reproducer i! in aconventional manner.

Referring now more particularly to the portion of the system embodyingthe present invention, the detector 95 includes an input circuitcomprising input-circuit terminals i8, i9 adapted to have appliedthereto from unit I I a frequency-modulated intermediate-frequencycarrier signal, the frequency of which deviates from a predeterminedmean or nominal frequency over a predetermined range of frequencydeviation in accordance with a modulation signal. The detector alsoincludes a vacuum tube 20 having two input electrodes or control grids2!, 22, an anode 23, a cathode 24, and a screen electrode or grid 25.There is included in the tube 20 means for effecting anode-currentsaturation thereof to render the detector I! substantially unresponsiveto amplitude variations of the carrier signal applied to the detector.This asaaaes means comprises an additional or space-charge electrode orgrid 28 adiacent the cathode which is positively energized from abiasing source indicated as +S.G.

The detector additionally includes means coupled to the input circuitcomprising terminals i8, is for deriving from the applied carrier signaltwo carrier signals having a relative phase which varies with thefrequency deviations of the applied carrier signal from a predeterminedfrequency and for individually applying the derived carrier signals tothe input electrodes 2 i 22. This means comprises an input transformer26 having a primary winding 21 coupled to the input terminals i8, i9through a condenser 28 and having a secondary winding 29, the primaryand secondary windings of transformer 28 being tuned by a. pair ofcondensers 80, 3|, respectively, to the nominal frequency of the appliedcarrier signal. The transformer 28 is thu double-tuned to the meanfrequency of the applied carrier signal and the signal potentialsdeveloped across transformer windings 2'! and 29 have a quadrature phaserelationship at this frequency. The relative phase relationship of thesesignal potentials, however, varies with the frequency of the appliedcarrier signal over its range of frequency deviation. In order that thedetector I! shall have an output which varies linearly with thefrequency deviations of the carrier signal applied thereto, it isnecessary that the relative phase of the carriersignal potentialsdeveloped across the transformer windings 21 and .29 shall vary linearlywith the frequency deviations of the applied carrier signal over a.predetermined range of'frequency deviation, the precise value of whichwill presently be considered in greater detail. This linearphase-frequency characteristic is obtained by reducing the Q, that is,the ratio of inductive or capacitive reactance to resistance, of thetuned circuits 21, 30 and 29, 3| to a sufllciently low value either byproportioning the constants of the circuit elements themselves, or bythe provision of damping resistors R, R connected in shunt to one orboth of the tuned circuits and shown in dotted lines for the reason thatthey may be comprised in whole or in part by the resistance of thecircuit elements 21, 29, 30 and ii, or by a combination of thesemethods.

The input electrode 2| of vacuum tube 20 is coupled to the primarywinding 21 to have applied thereto the carrier-signal potentialdeveloped across this winding. Similarly, the input electrode 22 iscoupled to the secondary winding 29 to have applied thereto thecarrier-signal potential developed thereacross. Means are provided forindividuallybiasing each of the input electrodes to a fixed operatingpotential, prefer-V ably on the linear portion of the operatingcharacteristic of each, comprising the sources of biasing potentialsindicated as C, C' which are connected to the input electrodes 2| and 22through the respective transformer windings 21 and 29. The screenelectrode 25 has a positive potential applied thereto from a sourceindicated as +Sc.

There is provided in the detector ii an output circuit coupled to theanode 23 of vacuum tube 20 for deriving therefrom a signal, theamplitude of which varies substantially only with the deviations offrequency of the applied carrier signal from its nominal frequency. Thisoutput circuit comprises output-circuit terminals 32, 33 and includes aload impedance comprising a resistor 34 in the anode circuit of thevacuum tube 20. The

anode 2! of vacuum tube is energized from a source of .space current,indicated as +B, through the resistor 34. The resistor 24 is bypassed toground for currents of carrier-signal frequency by a condenser 25effectively connected thereacross.

Considering now the operation of the circuit just described, andreferring to the curves of Fig. 2, a carrier signal is applied from theoutput circuit of the unit. I 4 to the input-circuit terminals 88. it ofthe detector I 5 to develop in the windings of the transformer 26 twocarrier signals having a relative phase which varies substantiallylinearly with the frequency of the applied an rier signal from itsnominal frequency. The transformer 26 thu comprises a frequencydiscriminator and provides for the input electrodes 2i, 22 of vacuumtube in two carrier signals which it derives from the applied carriersignal.

The space-charge grid 26 has a constant bias applied thereto from thesource +8.6. and is effective to produce a substantiallyconstant-intensity electrostatic field adjacent to cathode 24. therebyto cause the anode current of vacuum tube 20 to saturate at a relativelylow value of input-electrode voltage, as represented by the broken lineIs, Fig, 2, whenever both of the input electrodes 2i and 22 have aninstantaneous potential greater than a value en. The level of zero anodecurrent is represented in Fig. 2 by the broken line 0 corresponding toan input-electrode potential of e:. The bias of the input electrodes 2|and 22 is preferably adjusted in the following manner. The electrode 2|is biased to a large positive potential at which it normally w uld pro eanode-current saturation of tube 29 and the bias c' of electrode 22 isthen adjusted to a value midway between the values at which electrode 22biases tube 20 to anode-current cutofi' and anode=current saturation. Toadjust the bias of electrode 2|, electrode-ii is biased so farpositively that this electrode normally would cause anode-currentsaturationoi' tube 20 and the bias c is then adjusted to a valueapproximately midway between the values at which electrode 2| biasesvacuum tube 20, to anode-current cuton and anode-current saturation. Thecarrier signal applied to one of the input electrodes, for example theelectrode 2|, is represented by curve E21, and that applied to the otherinput electrode, for example the input electrode 22', is represented bycurve E22, it being assumedthat the applied carrier signal under theseconditiohshas its nominal value of friequeficy and, consequently, thatthe carrier signals applied to the input electrodes 2i and 22 have aquadrature phase dilference.

Eaohenf the input electrodes 2| and 22 is effective to bias the vacuumtube 20 to anode-cur-- rent cutoil when its instantaneous potential isless than the value ea. From this it will be evident that anodecurrentflows only during the intervals. when both of the inputelectrodes 2| and 22 have instantaneous potentials greater.

than the value er,- which condition occurs only during a relativelyshort interval of each cycle of the applied. carrier signal, asrepresented in Fig. 2 by the shaded area. The maximum value of the anodecurrent is limited to its saturation value Is and thus is substantiallyindependent of the maximum amplitudes of the positive halfcycles of thecarrier signals applied to the input electrodes 2| and 22. The minimumvalue of the anode current is, of course, limited byits zero valuerepresented by the, value 0 of Fig. 2.

and thus is independent of the maximum amplitudes of the negativehalf-cycles of the applied carrier signals. Consequently, it will beseen that the carrier-signal voltage developed across the resistor 34 inthe output circuit of the detector Iii does not vary substantially inamplitude with variations of amplitude of the applied carrier signal.

Assume now that the frequency of the applied carrier signal deviatesfrom its mean frequency. The phase diflerence between the carriersignals applied to the input electrodes 2i and 22 now changes by a phaseangle 45 from the quadrature phase relationship normally existingbetween these carrier signals and, assuming this phase change adds tothe initial quadrature phas difference as illustrated in Fig. 2, thearrier signal applied to the input electrode 22 under this condition isrepresented by the broken-line curve E'zz. It will be evident that theanode current of vacuum tube 20 now flows during a smaller interval ofeach cycle of the applied carrier signal. The following mathematicalanalysis of the detector operation indicates that the average value ofthe anode current, over one cycle of the applied carrier signal, varieslinearly with the phase change of the carrier signals applied to theinput electrodes 2| and 22 from their normal quadrature phaserelationship.

As a starting point for this analysis, it may be noted that the totalshaded area of Fig. 2 represents the integrated value of anode currentflowing during each cycle oi the applied carrier signal. It can be shownthat this area. and thus the integrated anode current, has very nearlythe value where:

Ip==th8 integrated value of anode current of tube 20, z' =theinstantaheous value of anode current of tube 20,

t=the period of the carrier signal applied to detector l5, and

Is=the value of anode current saturation of tube 29.

The average anode current during each cycl of the applied carrier signalis thus:

If has an value other than zero, the integrated value of anode currentis defined by the rela- I,,=J;z,dt=I,( (3) Thus, the average anodecurrent over one cycle of the applied carrier signal now has the value:

tion 2 a predetermined'range, for example 2.0 degrees,

with the frequency deviation of the carrier S18- nal applied to thedetector Ill. The output of the detector l thus varies linearly with thefrequency deviation of the carrier signal applied thereto andsubstantially independently of amplitude variations of the appliedcarrier signal, whereby the detector I5 is responsive only to thefrequency deviations of the applied carrier signal and is substantiallyunresponsive to amplitude variations thereof.

From the foregoing description of the Fig. l arrangement, it 'will beapparent that the spacecharge electrode 36 and source of energizing bias+S.G. therefor and the biasing sources -C, -C' for the controlelectrodes 2|, 22 comprise means for effecting anode-current saturationat a relatively low level of applied carrier-signal intensity andanode-current cutoif of the vacuum tube to render the detector i5substantially unresponsive to amplitude variations of the carrier signalapplied thereto for carrier-signal intensities, greater than such lowintensity level.

Fig. 3 is a circuit diagram representing a modifled form of theinvention which is essentially similar to the arrangement of Fig. 1,similar circuit elements being designated by similar reference numeralsand analogous circuit elements by similar reference numerals primed. Inthe Fig. 3 arrangement, the two carrier signals derived by thetransformer 26 are individually applied to the input electrode 2! and toa suppressor electrode 31 included in the vacuum tube 20'. Thisarrangement thus provides an alternative method of applying the twocarrier signals to input electrodes of the vacuum tube 20. Thearrangement and operation of the Fig. 3 modification are otherwiseessentially similar to that of Fig. 1 except for the feature that theportion of the screen electrode which is positioned between the inputelectrode 2| and the space-charge electrode 36 is effective with thelatter to control the level of anode-current saturation Is of the vacuumtube 20'. Consequently, the value of the potentials applied to thespace-charge electrode 36 from the source +S.G. and that applied to thescreen electrode 25 from the source +Sc are relatively proportioned toprovide the desired value of anode-current saturation Is. The operationis otherwise essentially similar to that of the Fig. 1 arrangement andwill not be repeated.

While in both the arrangements of Figs. 1 and 3, the means for derivingthe two carrier signals from that applied to the detector l5 comprisesthe transformer 25, it will be evident that any of the known forms offrequency discriminators other than the double-tuned transformer 26 maybe used by which to derive from the carrier signal applied to thedetector I5 two carrier signals having a relative phase which variessubstantially linearly with the frequency of the applied carrier signalover the range of frequency deviation of the latter.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseing, an input circuit adapted to have applied thereto a carrier signalthe frequency of which deviates over a predetermined range of frequencydeviation, a vacuum tube including two input electrodes and an anode,means coupled to said input circuit for deriving from said appliedcarrier signal two carrier signals having a relative phase which varieswith the frequency deviation of said applied carrier signal from apredetermined frequency and for individually applying said derivedcarrier signals to said input electrodes, means for effectinganode-current saturation of said tube to render said detectorsubstantially unresponsive to amplitude variations of said appliedcarrier signal, and an output circuit coupled to said anode for derivingtherefrom a signal the amplitude of which varies substantially only withdeviations of the frequency of said applied carrier signal from saidpredetermined frequency.

2. A carrier-signal frequency detector comprising, an input circuitadapted to have applied thereto a carrier signal the frequency of whichdeviates over a predetermined range of frequency deviation, a vacuumtube including two input electrodes and an anode, means coupled to saidinput circuit for deriving from said applied carrier signal two carriersignals having a relative phase which varies with the frequencydeviation of said applied carrier signal from a predetermind frequencyand for individually applying said derived carrier signals to said inputelectrodes, means for efiecting anode-current saturation andanode-current cutoff of said tube to render said detector substantiallyunresponsive to amplitude variations of said applied carrier signal, andan output circuit coupled to said anode for deriving therefrom a signalthe amplitude of which varies substantially only with deviations of thefrequency of said applied carrier signal from said predeterminedfrequency.

3. A carrier-signal frequency detector comprising, an input circuitadapted to have applied thereto a carrier signal the frequency of whichdeviates over a predetermined r'ange of frequency deviation, a vacuumtube including two input electrodes and an anode, means coupled to saidinput circuit for deriving from said applied carrier signal two carriersignals having a relative phase which varies with the frequencydeviation of said applied carrier signal from a predetermined frequencyand for individually applying said derived carrier signals to said inputelectrodes, means included in said tube for effecting anode-currentsaturation thereof to render said detector substantially unresponsive toamplitude variations of said applied carrier signal; and an outputcircuit coupled to said anode for deriving therefrom a signal theamplitude of which varies substantially only with deviations of thefrequency of said applied carrier signal from said predeterminedfrequency.

4. A carrier-signal frequency detector comprising, an input circuitadapted to have applied thereto a carrier signal the frequency of whichdeviates over a predetermined range of frequency deviation, a vacuumtube including two input electrodes, an additional electrode and ananode, means coupled to said input circuit for deriving from saidapplied carrier signal two carrier signals having a relative phase whichvaries with the frequency deviation of said applied carrier signal froma predetermined frequency and for individually applying said derivedcarrier signals to said input electrodes, means including saidadditional electrode for effecting anode-current saturation of said tubeto render said detector substantially unresponsive to amplitudevariations of said applied carrier signal, and an output circuit coupledto said anode for deriving therefrom a signal the amplitude of whichvaries substantially only with deviations of the frequency of saidapplied carrier signal from said predetermined frequency.

5. A carrier-signal frequency detector comprising, an input circuitadapted to have applied thereto a carrier signal the frequency of whichdeviates over a predetermined range of frequency deviation, a vacuumtube including two input electrodes, a space-charge electrode and ananode. means coupled to said input circuit for deriving from saidapplied carrier signal two carrier signals having a relative phase whichvaries with the frequency deviation of said applied carrier signal froma predetermined frequency and for individually applying said derivedcarrier signals to said input electrodes, means including saidspace-charge electrode for effecting anode-current saturation of saidtube to render said detector substantially unresponsive to amplitudevariations of said applied carrier signal, and an output circuit coupledto said anode for deriving therefrom a signal the amplitude of whichvaries substantially only with deviations of the frequency of saidapplied carrier signal from said predetermined frequency.

a A carrier-signal frequency detector comprising, an input circuitadapted to have applied thereto a carrier signal the frequency of whichdeviates over a predetermined range of frequency deviation, a vacuumtube including an anode, a cathode, two input electrodes, and aspace-charge grid adjacent said cathode and between said cathode andsaid input electrodes, means coupled to said input circuit for derivingfrom said applied carrier signal two carrier signals having a relativephase which varies with the frequency deviation of said applied carriersignal from a predetermined frequency and for individually applying saidderived carrier signals to said input electrodes, means including saidspace-charge grid and means for positively energizing said space-chargegrid for effecting anode-current saturation of said tube to render saiddetector substantially unresponsive to amplitude variations of saidapplied carrier signal, and an output circuit coupled to said anode forderiving therefrom a signal the amplitude of which varies substantiallyonly with deviations of the frequency of said applied carrier signalfrom said predetermined frequency.

7. A carrier-signal frequency detector comprising, an input circuitadapted to have applied thereto a carrier 1min the frequency of whichdeviates over a predetermined range of frequency deviation, a vacuumtube including two input electrodes and an anode, means coupled to saidinput circuit for deriving from said applied carrier signal two' carriersignals having a relative phase which varies with the frequencydeviation of said applied carrier simal from a predetermined frequencyand for individually applying said derived carrier signals to said inputelectrodes, means for efiecting anode-current saturation of said tube ata relatively low level of applied carrier-signal intensity to rendersaid detector substantially unresponsive to amplitude variations of saidapplied carrier signal in excess of said low-level intensity, and anoutput circuit coupled to said anode for deriving therefrom a theamplitude of which varies only with deviations of the frequency of saidapplied carrier signal from said predetermined frequency.

8. A carrier-signal frequency detector comprising, an input circuitadapted to have applied thereto a carrier signal the frequency of whichdeviates over a predetermined range of frequency deviation, a vacuumtube including two input electrodes and an anode, means coupled to saidinput circuit for deriving from said appliedcarrier signal two carriersignals having a relative phase which varies with the frequencydeviation of said applied carrier signal from a predetermined frequencyand for individually applying said derived carrier signals to said inputelectrodes, means for effecting anode-current saturation of said tube torender said detector substantially unresponsive to amplitude variationsof said applied carrier signal, means for fixedly biasing said inputelectrodes to points on substantially linear portions of the operatingcharacteristics of said vacuum tube, and an output circuit coupled tosaid anode for deriving therefrom a signal the amplitude of which variessubstantially only with deviations of the frequency of said appliedcarrier signal from said predetermined frequency.

9. A carrier-signal frequency detector comprising, an input circuitadapted to have applied thereto a carrier signal the frequency of whichdeviates over a predetermined range of frequency deviation, a vacuumtube including two input electrodes and an anode, means coupled to saidinput circuit for deriving from said applied carrier signal two carriersignals havin a relative phase which varies with the frequency deviationof said applied carrier signal from a predetermined frequency and forindividually applying said derived carrier signals to said inputelectrodes, means for effecting anode-current saturation andanode-current cutoff of said tube to render said detector substantiallyunresponsive to amplitude variations of said applied carrier signal,means for fixedly biasing said input electrodes to points on theoperating characteristics of said vacuum tubes substantially midwaybetween anode-current saturation and anode-cup rent cutoff of saidvacuum tube, and an output circuit coupled to said anode for derivingtherefrom a signal the amplitude of which varies substantially only withdeviations of the frequency of said applied carrier signal from saidpredeter mined frequency.

10. A carrier-signal frequency detector comprising, an input circuitadapted to have applied thereto a carrier signal frequency-modulatedover a predetermined range of frequency deviation in accordance with amodulation signal, a vacuum tube including two input electrodes and ananode. means coupled to said input circuit for deriving from saidapplied carrier signal two carrier signals having a relative phase whichvaries with the frequency deviation of said applied carrier signal froma predetermined frequency and for individually applying said derivedcarrier signals to said input electrodes, means for effectinganodecurrent saturation of said tube to render said detectorsubstantially unresponsive to amplitude variations of said appliedcarrier signal, and an output circuit coupled to said anode andincluding means for deriving from the average space current thereof saidmodulation signal.

JASPER J. OKRENT.

